1. Field of the Invention
This invention relates to apparatus for the acoustic detection of wind speed and direction at various altitudes.
2. Description of the Prior Art
Recently, for several reasons, there has been great interest in obtaining accurate estimations of variations in wind speed and direction at various altitudes and particularly systems which can give a continual and accurate reading of variations over relatively small discreet altitude variations over the first two or three thousand meters of altitude above ground.
Presently, there are several techniques which are available but these all suffer disadvantages.
The first is the use of instrumented towers. These can give a direct reading at whatever spacing is required but the towers need to be free standing so that their readings are not adversely effected by local buildings and, practically, it is difficult to use a tower to obtain readings for heights beyond 175 meters.
A second form is to track freely rising balloons. Such tracking is valuable where the situation up to very great heights is required. A good profile up to 30,000 meters can be obtained but the height resolution, of the order of 300 meters, is poor and, if a time resolution of a few minutes is required for extended periods, such a system can be prohibitively expensive.
It is also possible to use tethered kites, balloons or kitetoons but these have not been satisfactory for two reasons. Firstly if the system is tethered to more than a few hundred feet, they are hazardous to aircraft and the systems are, in any case difficult and expensive to operate and maintain over extended periods. Furthermore, kites are difficult to use in very light or very strong wind conditions and balloons and kitetoons are restricted to light or moderate wind conditions.
Because of recent developments and the necessity to maintain a close watch on wind speed at airports as landing speeds, particularly, are becoming critical, because there has been a great upsurge in interest in atmospheric pollution, and as there has been a deal of interest in wind power generating systems, a good knowledge of the wind parameters, particularly those at altitudes where aircraft can be at risk, where pollution will be held by an inversion and in selecting sites for wind power generating systems, the need for accurate and, preferably relatively portable, detectors of wind parameters are required.
For many years it has been known that the atmosphere can be remotely sensed by studying the back scattering or reflection of acoustic waves by irregularities in its temperature, humidity and velocity structures.
The interaction of the atmosphere with acoustic waves differs from that with electro-magnetic waves. Electro-magnetic waves are influenced by the temperature and humidity structures but this influence is very weak and difficult to detect, whereas it is possible to measure the strength of the back scattered signal from an acoustic wave, and hence the presence of small scale turbulance, with equipment which does not have to have the sensitivity or selectivity which would be necessary if an electro-magnetic wave was being studied.
Further, acoustic waves back scattered through 180.degree. are scattered by turbulence in the temperature profile but are not influenced by turbulence in the velocity structure.
There are two available techniques whereby the required information could be ascertained. The first of these is a bistatic geometric arrangement in which waves scattered approximately 150.degree. are observed so that the variations in the velocity structure contribute to the strength of the scattered signal and result in an improved signal to noise ratio. Two major disadvantages in bistatic systems are that they need very substantial land areas, if scattering up to an even moderate height is to be required, and the normal use of multiple receiving and/or transmitting antennae and the cabling requirements associated therewith make the system excessively expensive.
Also, if it is necessary to use a full spectral analysis the system has to be associated with a relatively complex computer and, again, costs are high.